Physics Faculty Works Seaver College of Science and Engineering 12-21-2010 Does Entropic Gravity Bound The Masses Of The Photon And Graviton? Jonas R. Mureika Loyola Marymount University, [email protected] R. B. Mann University of Waterloo Follow this and additional works at: https://digitalcommons.lmu.edu/phys_fac Part of the Physics Commons Recommended Citation Mureika, J.R., & Mann, R.B. (2011). Does Entropic Gravity Bound the Masses of the Photon and Graviton? Modern Physics Letters A, 26(3), 171-181. This Article - pre-print is brought to you for free and open access by the Seaver College of Science and Engineering at Digital Commons @ Loyola Marymount University and Loyola Law School. It has been accepted for inclusion in Physics Faculty Works by an authorized administrator of Digital Commons@Loyola Marymount University and Loyola Law School. For more information, please contact [email protected]. Does Entropic Gravity Bound the Masses of the Photon and Graviton? J. R. Mureika1 Department of Physics, Loyola Marymount University, Los Angeles, CA and R. B. Mann2 Perimeter Institute for Theoretical Physics and Department of Physics and Astronomy, University of Waterloo, Waterloo, ON Canada Abstract If the information transfer between test particle and holographic screen in entropic gravity respects both the uncertainty principle and causality, a lower limit on the number of bits in the universe relative to its mass may be derived. Furthermore, these limits indicate particles that putatively travel at the speed of light – the photon and/or graviton – have a non-zero mass m ≥ 10−68 kg. This result is found to be in excellent agreement with current experimental mass bounds on the graviton and photon, suggesting that entropic gravity may be the result of a (recent) softly-broken local symmetry. Stronger bounds emerge from consideration of ultradense matter such as neutron arXiv:1005.2214v2 [gr-qc] 21 Dec 2010 stars, yielding limits of m ≥ 10−48 − 10−50 kg, barely within the experimental photon range and outside that of the graviton. We find that for black holes these criteria cannot be satisfied, and suggest some possible implications of this result. 1Email: [email protected] 2Email: [email protected] 1 Introduction The recent proposal [1, 2, 3] that gravity is an emergent phenomenon of entropy maximization has added to a growing list of gravitational-thermodynamic dualities [4, 5, 6, 7, 8, 10, 11, 12, 13], raising much interest amongst the theoretical physics community. It can be understood as an application of the holographic principle, which states that there is a duality between a physical de- scription (including gravity) of a volume of space and a corresponding (non-gravitational) physical theory formulated on the boundary this volume. While the motivation behind studying these dualites stems from a desire to better understand quantum gravity, cosmological considerations of holographic duality have been of some interest for a number of years [14, 15, 16]. Further progress came with with derivation of the Friedmann equations from the first law of thermodynamics [17] on the apparent cosmic event horizon, along with the assumptions that the entropy is proportional to its area and temperature to its surface gravity. The advent of entropic gravity [3] has prompted theorists to deploy this novel framework not only in quantum gravity [18, 19, 20] and quantum information [21, 22, 23], but also to explain a wide range of cosmological phenomena, including implications for black hole temperature [24, 25, 26, 27], dark energy [28, 29, 30, 31, 32], and inflation [33, 34, 35, 36]. Entropic gravity, and hence holography, thus plays a deep role in connecting these two bookend- realms of universal scales. Cosmological holography is essentially the proposal that all of the in- formation in our universe is encoded in a structure on its cosmological horizon. An application of this proposal has been put forth in a recent conjecture by Smoot [37], that all possible past and future histories of the universe are encoded on its apparent horizon, thereby making a connection. Here we proceed along similar lines, asking how much the “whole” – the total mass and in- formation content of the Universe – can tell us about the “parts” – the lightest possible mass of elementary particles. Specifically we point out that there is a lower limit to the number of bits of a holographic screen in entropic gravity provided the information transfer between a test particle and the screen respects both the uncertainty principle and causality. When applied to the entire universe (i.e. taking the holographic screen to be the boundary of the visible Universe), this limit indicates that all quanta have a non-zero mass m ≥ 10−68 kg. This implies that the exact symme- tries governing the behaviour of photons and gravitons are broken, albeit very softly. This lower bound is only a few orders of magnitude below current experimental bounds on the masses of these particles. It is also weakly time-dependent on cosmic time scales, suggesting possible tests of cosmological holography and entropic gravity. We also consider implications of this bound for holographic screens due to ultradense matter (such as neutron stars) and black holes. For both neutron stars and stellar-mass black holes the bound is larger but remarkably consistent at m ≥ 10−48 − 10−50 kg. While barely within the experimental range of the photon mass, such a value excludes current inferred bounds on the mass of the graviton. This result seems to suggest that either the method discussed herein is either not applicable to the graviton (or perhaps to gauge particles in general), or alternatively that the graviton is not an actual particle in the entropic gravity scenario. Taking a black hole to be the source of the holographic screen, we find that the bound is inversely proportional to the black hole radius. Hence for a sufficiently small black hole (smaller than about a solar mass) the bound cannot be empirically satisfied. If massless quanta cannot be accommodated within entropic gravity, then this suggests a minimal bound on the mass of a black hole. 2 Entropic Gravity Primer An entropic force is one that drives a system’s entropy to increase [3], ∆S F = T . (1) entropic ∆x The foundation of the idea [3] that gravitation is such a force relies on a holographic argument relating the entropy to the area of a screen, the temperature to the acceleration of the particle, and the thermodynamic equipartition theorem. A test particle of mass m is located some distance from another (presumably larger) mass M, the latter of which generates a holographic screen at a distance r. On this screen, the holographic information from mass M is encoded as A 4πr2c3 N = = (2) 2 ~ ℓP G bits. As the mass m approaches the screen, its own entropy bits begin to transfer to the screen, and it is this increase in screen entropy that generates an attractive force. Specifically, the entropy transferred by m at a distance ∆x is ∆x h ∆S = 2πk , Ż = (3) B Ż mc so that a “quantized” unit of entropy ∆S = 2πkB is incremented when the particle is within a distance equal to its Compton wavelength. The energy on the screen obeys thermal equipartition, 1 2Mc2 E = Mc2 = Nk T −→ k T = (4) 2 B B N Substituting (2) and (3) into (1) yields Newton’s universal law of gravitation, GmM F = − (5) entropic r2 which carries with it the novelty that the force is emergent instead of fundamental.3 3The minus sign signifies an attractive force, which results from the sign of the term ∆x used in the calculation of the entropic force actually being negative [3]. 3 Bounding Information Transfer The formulation of the entropy mechanism suggests that there is an inherent uncertainty ∆x in the location of the test mass m relative to the holographic screen. Indeed, when ∆x ∼ λc, the entropy of m merges with that of the screen. This leads one to suggest that, when the position uncertainty of m is ∆x, there is a statistical fluctuation of the screen’s entropy ∆S , and hence an uncertainty in its energy ∆E ∼ T∆S that must abide by quantum mechanical considerations4 A naive limit on the variation in momentum of the mass m may be derived from the standard Heisenberg Uncertainty Principle (HUP), ~ mc ∆p ≥ = . (6) 2∆x 4π c The uncertainty in the velocity is thus ∆v ≥ 4π , but is really inconsequential to the problem at hand as it is the average velocity that must respect v < c. Furthermore, the uncertainty principle further suggests that the fluctuation in the energy of the screen is constrained to occur during the interval ~ ~ ∆E ∆t ≥ =⇒ ∆t ≥ (7) 2 ∆2E Once the particle is within a Compton wavelength of the screen, the “speed” of information transfer ∆x is on the order of vI ∼ ∆t , which according to Equation 7 becomes ∆x 2Ż∆E v ∼ ≤ (8) I ∆t ~ 4Several authors have previously addressed the connection between entropic gravity and generalized position- momentum uncertainty principles [38, 39]. Following the rationale applied in Section 2, the uncertainty in the energy may be written 4πMc2 ∆E = T∆S = 2πk T = (9) B N and thus 16π2 M vI ≤ · c (10) N m Imposing a causality bound on this upper limit yields 16π2 M 16π2 M · c ≤ c =⇒ · < 1 (11) N m N m This naive approach is fraught with ambiguity, however, due to the well-known fact that time itself is a parameter and not associated with any hermitian operator. A clarification was offered by Mandelstam and Tamm [40], who modified eq. (7) via an auxiliary observable O, obtaining ∆O ~ ∆E ≥ . (12) dO 2 dt relating the standard deviation of the energy operator of some non-stationary state to the standard dX deviation ∆O.